Assessment of Multiple GNSS Real-Time SSR Products from Different Analysis Centers

International Journal of Geo-Information

Article Assessment of Multiple GNSS Real-Time SSR Products from Different Analysis Centers

Zhiyu Wang 1,2, Zishen Li 1,*, Liang Wang 1,2,*, Xiaoming Wang 1,3 ID and Hong Yuan 1

1 Academy of Opto-Electronics, Chinese Academy of Sciences, 9 Dengzhuang South Road, Haidian District, Beijing 100094, China; [email protected] (Z.W.); [email protected] (X.W.); [email protected] (H.Y.) 2 University of Chinese Academy of Sciences, 19A Yuquan Road, Shijingshan District, Beijing 100049, China 3 School of Environment and Spatial Informatics, China University of Mining and Technology, 1 Daxue Road, Xuzhou 221116, China * Correspondence: [email protected] (Z.L.); [email protected] (L.W.); Tel.: +86-10-8217-8896 (Z.L. & L.W.)

Received: 15 January 2018; Accepted: 7 March 2018; Published: 8 March 2018

Abstract: The real-time State Space Representation (SSR) product of the GNSS (Global Navigation Satellite System) orbit and clock is one of the most essential corrections for real-time precise point positioning (PPP). In this work, the performance of current SSR products from eight analysis centers were assessed by comparing it with the ﬁnal product and the accuracy of real-time PPP. Numerical results showed that (1) the accuracies of the GPS SSR product were better than 8 cm for the satellite orbit and 0.3 ns for the satellite clock; (2) the accuracies of the GLONASS (GLObalnaya NAvigatsionnaya Sputnikovaya Sistema) SSR product were better than 10 cm for orbit RMS (Root Mean Square) and 0.6 ns for clock STD (Standard Deviation); and (3) the accuracies of the BDS (BeiDou Navigation Satellite System) and Galileo SSR products from CLK93 were about 14.54 and 4.42 cm for the orbit RMS and 0.32 and 0.18 ns for the clock STD, respectively. The simulated kinematic PPP results obtained using the SSR products from CLK93 and CLK51 performed better than those using other SSR products; and the accuracy of PPP based on all products was better than 6 and 10 cm in the horizontal and vertical directions, respectively. The real-time kinematic PPP experiment carried out in Beijing, Tianjin, and Shijiazhuang, China indicated that the SSR product CLK93 from Centre National d’Etudes Spatiales (CNES) had a better performance than CAS01. Moreover, the PPP with GPS + BDS dual systems had a higher accuracy than those with only a GPS single system.

Keywords: precise point positioning; orbits and clocks; state space representation; analysis centers

1. Introduction Precise Point Positioning (PPP) is one of the most widely-used approaches for high-precision real-time positioning with the development of multi-frequency global navigation satellite systems (GNSS). However, the PPP approach relies heavily on the availability of the high-precision satellite orbit and clock corrections [1–9]. Currently, the International GNSS Service (IGS) agency and various analysis centers (ACs) provide users with precise satellite orbit and clock products through FTP (File Transfer Protocol) in three forms: ultra-rapid, rapid, and final [10–12]. The rapid and final orbit/clock products are available after around 17 h after the end of the previous UTC (Coordinated Universal Time) day and 13 days after the end of the solution week, respectively, which mean that they cannot be used for real-time applications [13]. Although ultra-rapid products are available for real-time applications, its accuracy is not good enough for high-precision PPP. Each ultra-rapid orbit file usually covers 48 h, but only the first 24 h of the orbit are generated using actual observations and the second 24 h are extrapolated using the first 24-h orbit.

ISPRS Int. J. Geo-Inf. 2018, 7, 85; doi:10.3390/ijgi7030085 www.mdpi.com/journal/ijgi ISPRS Int. J. Geo-Inf. 2018, 7, 85 2 of 20

To meet the growing needs for real-time high-precision positioning and application, IGS founded a Real-Time Working Group in 2002 committed to the construction of infrastructure and to set up standards as well as technical specifications related to high-precision real-time GNSS [14]. In 2007, IGS started the Real-Time Pilot Project (RTPP) and has extended its capability to support applications requiring real-time access to IGS products since 2013 by providing GPS and GLONASS dual-system orbit and clock corrections based on RTCM (Radio Technical Commission for Maritime Services) and NTRIP (Networked Transport of RTCM via Internet Protocol) [15–21]. Multi-GNSS real-time orbit and clock products are also making headways with the development of BDS and Galileo. Currently, there are a wide collection of real-time orbit and clock products for either GPS or GPS + GLONASS, developed by ACs such as BKG (Bundesamt für Kartographie und Geodäsie), CNES (Centre National d’Etudes Spatiales), DLR (Deutsches Zentrum für Luft- und Raumfahrt), ESA (European Space Agency), GFZ (Deutsches GeoForschungsZentrum), and GMV (GMV Aerospace and Defense). CNES was the first to provide RTS for all four systems (GPS/GLONASS/BDS /GALILEO) since 2015 (IGS MAIL 7183). Moreover, China started the construction of the international GNSS Monitoring and Assessment System (iGMAS) in 2012. The main task of iGMAS was to (1) establish a worldwide near-real-time tracking network for BDS, GPS, GLONASS, and Galileo; (2) build an information service platform for data collection, storage, analysis, management, and publication; (3) monitor and assess the operation status and key performance indicators of all GNSS [22]. At present, there are 30 global tracking stations, three data centers and eight analysis centers that can provide precise products to support satellite navigation technology testing, monitoring assessment, scientific research, and various applications [23].The Institute of Geodesy and Geophysics (IGG) of the Chinese Academy of Sciences (CAS), one of the iGMAS analysis centers, is also beginning to develop multi-system real-time orbit and clock correction products with the advance of iGMAS.

2. The Acquisition of Real-Time Observation Data and State Space Representation Product It is critical for the real-time PPP to access real-time data and SSR products in an efficient way [8,11,16,18,21,24]. Accessing GNSS data via the Internet based on NTRIP has been widely used in many applications. For instance, it has been adopted in data transmission between CORS (Continuously Operating Reference Stations) servers and receivers. The NTRIP agreement, which officially became an RTCM standard in November 2004, is used for sending data streams in the format of RTCM 2.0 and 3.0. Real-time orbit and clock correction data generated by ACs in IGS-RTPP are released in SSR (State Space Representation) format in compliance with the RTCM standard and broadcasted via NTRIP (RTCM 2011). Figure1 shows the broadcasting, receiving, and precise positioning process of real-time GNSS data/products. The GNSS data transmission system based on NTRIP generally consists of four parts: the data source, server (NtripServer), broadcaster (NtripCaster), and client terminal (NtripClient). The table of data sources generated by NTRIP broadcasters contains general information about data sources including their ID, RTCM version, data type, etc. One can access this table via the Internet on a client terminal, and select proper mount points to obtain raw data or corrections from NTRIP data sources with a short latency [25]. BKG Ntrip Client (BNC) is one of the most widely-used software package for obtaining real-time data and products [20], but it only supports data decoding in the RTCM format, making it difficult to broadcast real-time products in the latest format. In light of this, the GNSS research group at the IGG of Chinese Academy of Sciences has developed an alternative software named IGG-Ntrip, which presents the following features: • Supports both RTCM and iGMAS formats; • Supports real-time data and products for four systems (GPS/GLONASS/BDS/GALILEO) with multiple addresses and mount points; • Provides a data sharing mechanism based on sharing memory and Socket; • A user-friendly graphic interface that allows users to select stations on a map; ISPRS Int. J. Geo-Inf. 2018, 7, 85 3 of 20

ISPRSAll Int. theJ. Geo-Inf. real-time 2018, 7 data, x FOR and PEER products REVIEW used in this work were obtained via IGG-Ntrip. 3 of 19

Figure 1. Broadcasting, receiving, receiving, and and precise position positioninging process of real-time Global Navigation Satellite System (GNSS) data/products.

3. Real-Time Precise Orbit and Clock Recovery 3. Real-Time Precise Orbit and Clock Recovery The real-time data streams from IGS/ACs provide corrections of orbits and clocks to broadcast The real-time data streams from IGS/ACs provide corrections of orbits and clocks to broadcast ephemeris. As above-mentioned, these corrections are essential for obtaining high-precision orbits ephemeris. As above-mentioned, these corrections are essential for obtaining high-precision orbits and and clocks for precise real-time PPP [10,26–28]. clocks for precise real-time PPP [10,26–28]. Real-time orbit corrections are provided in radial, along-track, and cross-track directions in a Real-time orbit corrections are provided in radial, along-track, and cross-track directions satellite-fixed coordinate system. Thus, it was necessary to first convert orbit corrections into an in a satellite-fixed coordinate system. Thus, it was necessary to first convert orbit corrections Earth-Fixed reference frame (ECEF) system, which was adopted for the positioning. RTCM-SSR into an Earth-Fixed reference frame (ECEF) system, which was adopted for the positioning. corrections included the following parameters [29–31]: RTCM-SSR corrections included the following parameters [29–31]: Δ=δδδδδδ''' ssr(t,IODE)(O,O,O,O,O,O,C,C,C,) 0 r a c r0 a0 0 c 0 1 2 (1) ∆ssr(t0, IODE) = (δOr, δOa, δOc, δOr, δOa, δOc,C0,C1,C2), (1) δδδδδδ''' where IODE is the issue of data; O,racrac O, O, O,0 0 O,0 O are the corrections and rate of change where IODE is the issue of data; δOr, δOa, δOc, δOr, δOa, δOc are the corrections and rate of change at at time t in the radial, along-track, and cross-track directions, respectively; C,C,C are the time t0 in the0 radial, along-track, and cross-track directions, respectively; C0, C1, C2 are012 the polynomial polynomialcoefficients forcoefficients calculating for clockcalculating corrections, clock respectively.corrections, respectively. h T iT δδδ δ The orbit correctioncorrection δ == racδr δa δc at atepoch epoch t cant can be be calculated calculated using using Equation Equation (2) (2) based based on the SSR product      0  δr δOr δOr δδ  δ' 0 =   =rrr OO +  ( − ) δ  δa  δOa   δOa  t t0 (2) δδ==O+Ott δ δ' （-0 ） δc aaa0δOc δOc (2) δδOO  δ' After converting the corrections fromccc the satellite-fixed  system to the ECEF system in X, Y, and ZAfter directions, converting the precise the corrections orbit R can from be the calculated satellite- usingfixed Equation system to (3), the ECEF system in X, Y, and Z directions, the precise orbit R can be calculated  using Equation (3),  X xb δr h → → → i =  =    − δ   R Y Xxybb er ea ec r δa (3)         RYZ ==z y − eeeδ δc  bb rac  a  (3)    δ  Zz b  c   where e,e,erac are the unit vectors in the radial, along-track, and cross-track directions, T  respectively; x= xbbb y z is the satellite orbit calculated from the broadcast ephemeris.

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→ → → where er, ea, ec are the unit vectors in the radial, along-track, and cross-track directions, respectively; h iT x = xb yb zb is the satellite orbit calculated from the broadcast ephemeris. It should be noted that there are two types of reference points for satellite position corrections in the SSR messages provided by NTRIP: APC (antenna phase center) and COM (center of mass). The data source table provided by the NTRIP casters indicates which reference point is used. If APC is adopted, the antenna phase bias correction shall be taken into account to obtain the coordinates of the satellite’s center of mass under ITRF. Regarding the recovery of precise clocks, Equation (4) is used to determine the clock correction ∆k at epoch t with the polynomial coefﬁcients C0,C1,C2 given at the reference epoch t0.

2 ∆k = C0 + C1(t − t0) + C2(t − t0) (4)

Then, the precise satellite clock ∆ts at epoch t can be calculated with the following equation,

∆k ∆ts = ∆tb − (5) Vc where Vc is the speed of light in the vacuum; and ∆tb is the clock correction generated from the broadcast ephemeris.

4. Assessment of the Real-Time Orbit and Clock Corrections for Multi-GNSS The precision of orbit and clock corrections is a critical issue for high-precision positioning using PPP. In this study, the satellite orbits and clocks were calculated epoch-by-epoch using the above-mentioned method with the multi-GNSS SSR products provided by eight selected ACs. The results were then compared against the final precise products released by IGS/ACs to assess the accuracy of these multi-system real-time orbit and clock corrections. The real-time products used in this study were developed by IGS, BKG, DLR, ESA, GFZ, GMV, CNES, and CAS, as listed in Table1 in detail. It can be seen from Table1 that all products supported GPS, while the products CLK93 and CAS01 supported BDS and Galileo. The data used in this experiment were collected from 12 to 18 September 2017 with a sampling interval of 30 s. The final products released by IGS and ESA were selected as references for assessing the performance of real-time SSR products for GPS and GLONASS systems, respectively, while for BDS and Galileo, the final products of GBM (Geodetic Benchmark) released by GFZ were used as references.

Table 1. The details of the real-time products used in this work.

ACs IGS BKG DLR ESA GFZ GMV CNES CAS Products IGS03 CLK10 CLK20 CLK51 CLK70 CLK80 CLK93 CAS01 System G+R G G+R G G+R G+R G+R+C+E G+R+C+E

The RMS of differences between orbits calculated using real-time corrections and final products was calculated for the radial (R), along-track (A), and cross-track (C) directions in a satellite-fixed coordinate system with the equation s n ∆2 RMS = ∑ i (6) i=1 n where ∆i represents the orbit mutual differences in nodes; and n is the number of mutual differences. For clock comparison, the RMS and STD of the differences between the real-time clock and the reference clock are generally calculated for the assessment after unifying statistical criteria, where RMS reflects the compliance of clock correction and pseudo-range, while STD represents the real resolution precision of clock correction which has a great influence on processing phase data [21,32]. Meanwhile, we considered the mean of the clocks for all the satellites as zero as the datum instead of selecting a ISPRS Int. J. Geo-Inf. 2018, 7, 85 5 of 20 reference satellite as the datum [3,28], which may cause the loss of precision for the reference satellite. ISPRS Int. J. Geo-Inf. 2018, 7, x FOR PEER REVIEW 5 of 19 The adopted mathematical approach is as follows:

 Δ=clkj jj clkj − clk jj −Δ t  ∆clk i= clk rt,i− clk pt,i− i ∆ti   i = rt,i pt,i  js=  in  j=s   jji=n j j  Δ= Δ j  =  j (clk)/ni  sum sum= rtclk clk rt,i  = ( )  rti ∑i rt,i  ∆ i1= ∑ ∆clki /n   = j1=   =  j 1  i =1  js=  ins  =  i=n j 2 j s j Δ j 2 = j (clk)∑ (∆iclk ) (7) sumpt clk pt ,i i (7) sumpti = ∑i clk j j i1= i=1  j1= pt,i  RMSRMS==  j=1  n  sum −sum−  s n  sumrti rtpt sumi  pt i=n 2  ∆ti =Δ= ii in= j j ti s  ∑ (∆jjclki−∆2 ) s  (clkΔ−Δ= )   STDj = i 1 i  STDj = i1= n  n where i and j are the epoch and the satellite number; n is the sum of epochs; s is the number of satellites; j where i jand j are the epoch and the satellite number; n is the sum of epochs; s is the number of clkrt,i and clkpt,i are the real-time and post-processing reference clock corrections; ∆ti is the datum satellites; clkj and clkj are the real-time and post-processing reference clock corrections; Δt difference betweenrt,i the two clockpt,i correction products with the selected datum; RMSj and STDj arei the j statisticalis the indicators datum difference of RMS between and STD, the respectively.two clock correction products with the selected datum; RMS and STDj are the statistical indicators of RMS and STD, respectively. 4.1. Validation Results of GPS Real-Time Orbit and Clock Products 4.1. Validation Results of GPS Real-Time Orbit and Clock Products Using the IGS ﬁnal product as the reference, the RMS of the differences of the GPS real-time orbit products fromUsing thethe eightIGS final selected product ACs as arethe shownreference, in th Figuree RMS2, of where the differences the RMS of the GPS orbit real-time differences were calculatedorbit products in the from R, A,the andeight C directions,selected ACs respectively, are shown in as denotedFigure 2, bywhere the red,the RMS green, of andthe blueorbit bars; differences were calculated in the R, A, and C directions, respectively, as denoted by the red, green, the RMS and STD of the differences of the GPS real-time clock products from the eight selected ACs are and blue bars; the RMS and STD of the differences of the GPS real-time clock products from the eight shown by the blue and red bars in Figure3. The average accuracies of the orbit and clock corrections selected ACs are shown by the blue and red bars in Figure 3. The average accuracies of the orbit and for eachclock product corrections over for all each the product GPS satellites over all are the givenGPS satellites in Table are2. given in Table 2.

Figure 2. The RMS of the differences of real-time GPS orbit between the eight selected ACs’ products Figure 2. The RMS of the differences of real-time GPS orbit between the eight selected ACs’ products and the IGS final product. and the IGS ﬁnal product.

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Figure 3. The Root Mean Square (RMS) (blue bar) and Standard Deviation (STD) (red bar) of the Figure 3. The Root Mean Square (RMS) (blue bar) and Standard Deviation (STD) (red bar) of the differences of real-timereal-time GlobalGlobal PositioningPositioning SystemSystem (GPS) clockclock betweenbetween thethe eighteight selectedselected analysisanalysis centers’centers’ (ACs’)(ACs’) productsproducts andand thethe InternationalInternational GNSSGNSS ServiceService (IGS)(IGS) ﬁnalfinal product.product.

Table 2. The average accuracies of GPS real-time orbit and clock products from the eight selected ACs Table 2. The average accuracies of GPS real-time orbit and clock products from the eight selected ACs withwith respectrespect toto thethe IGSIGS ﬁnalfinal productsproducts duringduring thethe testtest period.period. Orbits (cm) Clocks (ns) Products Orbits (cm) Clocks (ns) Products R A C 1DRMS RMS STD IGS03 5.79 R8.04 A7.02 C 1DRMS 7.01 RMS STD 0.35 0.19 CLK10 IGS03 2.76 5.794.17 8.04 3.56 7.02 7.01 3.54 0.35 0.19 0.79 0.29 CLK20 CLK10 2.48 2.763.97 4.17 2.94 3.56 3.54 3.19 0.79 0.29 0.43 0.12 CLK20 2.48 3.97 2.94 3.19 0.43 0.12 CLK51 2.09 3.29 2.74 2.75 0.46 0.13 CLK51 2.09 3.29 2.74 2.75 0.46 0.13 CLK70 CLK70 5.14 5.147.46 7.46 6.11 6.11 6.31 6.31 0.32 0.18 0.32 0.18 CLK80 CLK80 5.05 5.057.19 7.19 5.95 5.95 6.13 6.13 0.31 0.17 0.31 0.17 CLK93 CLK93 2.19 2.193.50 3.50 2.79 2.79 2.88 2.88 0.45 0.14 0.45 0.14 CAS01 CAS01 5.01 5.016.77 6.77 5.67 5.67 5.86 5.86 0.69 0.19 0.69 0.19

ItIt cancan bebe seenseen from from Table Table2 that2 that the the accuracies accuracies of theof the GPS GPS orbit orbit product product in the in radialthe radial direction direction was muchwas much better better than thatthan in that the along-trackin the along-track and cross-track and cross-track directions. directions. The CLK51 The performed CLK51 performed the best in the 1D-RMSbest in the comparison, 1D-RMS comparison, i.e., 2.09, 3.29, i.e., and 2.09, 2.74 3.29, cm and in the2.74 radial, cm in along-track,the radial, along-track, and along-cross and along-cross directions, respectively,directions, respectively, while the IGS03 while was the the IGS03 worst, was i.e., the 5.79, worst, 8.04, i.e., and 5.79, 7.02 cm,8.04, again and in7.02 those cm, three again directions. in those Regardingthree directions. the GPS Regarding real-time the clock GPS product, real-time all clock the product, RMS and all STD the wereRMS betterand STD than were 0.5 andbetter 0.2 than ns, except0.5 and for 0.2 CLK10 ns, except and CAS01.for CLK10 The and real-time CAS01. GPS The clock real-time product GPS from clock CLK80 product was thefrom best CLK80 with anwas RMS the ofbest 0.31 with ns an and RMS STD of of 0.31 0.17 ns ns, and while STD the of CLK100.17 ns, waswhile the the worst CLK10 one was with the an worst RMS one of 0.79 with ns an and RMS STD of 0.79 ns and STD of 0.29 ns. Overall, the real-time GPS orbit and clock product from CLK51 and CLK93 basically had the same accuracy, i.e., the 1D-RMS of the orbit product was better than 3.0 cm and the

ISPRS Int. J. Geo-Inf. 2018, 7, 85 7 of 20 of 0.29 ns. Overall, the real-time GPS orbit and clock product from CLK51 and CLK93 basically had the same accuracy, i.e., the 1D-RMS of the orbit product was better than 3.0 cm and the STD of the clock product was better than 0.15 ns. The differences in accuracies between the different ACs may have resulted because of the different distributions of contributed stations and the different strategies adopted by CLK51 and CLK93; however, these differences generally do not affect the real-time PPP achieving a positioning with about a 5–10 cm accuracy. Aside from the accuracy of the products from different ACs, it should be noted that the latency of the real-time SSR corrections is also an important aspect that should be taken into account in the quality assessment since the positioning accuracy will decrease with the increasing latency of the real-time SSR corrections [33]. It can be seen from Table3, that the CLKxx products generally had a latency of about 4–10 s, but the latency for the IGS03 products was around 30 s. Meanwhile, we also analyzed different generations of GPS satellites (BLOCKIIR-A, BLOKII-B, BLOKIIR-M, BLOKIIF) using the same product, but we did not ﬁnd anything particularly regular. Thus, we were able to conclude that the accuracy of orbits and clock products had no obvious correlation with different satellite generations.

Table 3. The latency of the real-time State Space Representation (SSR) corrections from different ACs.

Products Orbit/Clock Update Interval Latency of Orbit/Clock CLK10 60 s/5 s 5 s CLK20 5 s/5 s 4 s CLK51 5 s/5 s 6 s CLK70 10 s/5 s 6 s CLK80 5 s/5 s 6 s CLK93 5 s/5 s 8 s CAS01 1 s/1 s 8 s IGS03 60 s/10 s 28 s

4.2. Validation Results of GLONASS Real-Time Orbit and Clock Products Different from the real-time GPS orbit and clock products, the GLONASS products are only broadcasted in IGS03, CLK20, CLK70, CLK80, CLK93, and CAS01. The performance of GLONASS products is validated by comparing them with the ESA ﬁnal product. The accuracies of GLONASS orbits are also given in the radial, along-track, and along-cross directions shown in Figure4, and the accuracies of the GLONASS clock products from the selected ACs are given in Figure5. The average accuracies of the GLONASS satellite orbit and clock products during the test period are illustrated in Table4. It should be noted that the satellite orbits of R12 and R14 were absent in the products from IGS03, CLK70, CLK 80, CLK93, and CAS01 in the test period, and the result of R8 and R12 were also absent in the products from CLK20 and CLK80, respectively.

Table 4. The average accuracies of GLObalnaya NAvigatsionnaya Sputnikovaya Sistema (GLONASS) orbit and clock products from the six selected ACs compared with the IGS ﬁnal products during the test period.

Orbits (cm) Clocks (ns) Products R A C 1DRMS RMS STD IGS03 5.34 8.74 6.95 7.15 9.38 0.50 CLK20 4.45 7.09 5.69 5.84 8.29 0.31 CLK70 6.21 8.55 7.73 7.56 1.74 0.17 CLK80 6.44 9.02 7.46 7.71 0.82 0.33 CLK93 4.15 7.16 5.32 5.68 2.97 0.21 CAS01 10.33 34.84 20.57 24.11 5.49 1.86 ISPRS Int. J. Geo-Inf. 2018, 7, 85 8 of 20 ISPRS Int. J. Geo-Inf. 2018, 7, x FOR PEER REVIEW 8 of 19 ISPRS Int. J. Geo-Inf. 2018, 7, x FOR PEER REVIEW 8 of 19

Figure 4. The RMS of the differences of real-time GLONASS orbit between the six selected ACs’ FigureFigure 4. 4.The The RMS RMS of of the thedifferences differences ofof real-timereal-time GL GLONASSONASS orbit orbit between between the the six six selected selected ACs’ ACs’ products and the European Space Agency (ESA) final product. productsproducts and and the the European European Space Space Agency Agency (ESA) (ESA) ﬁnalfinal product.product.

Figure 5. The RMS (blue bar) and STD (red bar) of the differences of real-time GLONASS clock Figure 5. The RMS (blue bar) and STD (red bar) of the differences of real-time GLONASS clock Figurebetween 5. The the RMS six selected (blue bar) ACs’ and prod STDucts (red and bar) the of IGS the final differences product. of real-time GLONASS clock between thebetween six selected the six ACs’ selected products ACs’ and prod theucts IGS and ﬁnal the product.IGS final product. It can be seen from Table 4 that the average accuracy (1D-RMS) of the real-time GLONASS It can be seen from Table 4 that the average accuracy (1D-RMS) of the real-time GLONASS satellite orbit from all the ACs was almost at the same level (about 5–8 cm) except for the product satelliteIt can beorbit seen from from all Tablethe ACs4 that was the almost average at accuracythe same (1D-RMS)level (about of the5–8 real-timecm) except GLONASS for the product satellite from CAS01. Except for IGS03 and CAS01, the STDs of the real-time GLONASS satellite clock orbitfrom from CAS01. all the Except ACs was for almostIGS03 atand the CAS01, same level the (aboutSTDs of 5–8 the cm) real-time except forGLONASS the product satellite from clock CAS01. products from the other ACs were about 0.2–0.3 ns. Regarding the RMS of the GLONASS satellite Exceptproducts for IGS03 from andthe CAS01,other ACs the were STDs about of the 0.2–0.3 real-time ns. GLONASSRegarding satellitethe RMS clock of the products GLONASS from satellite the other clock, although the different references in the clock products from different ACs was considered ACsclock, were although about 0.2–0.3 the different ns. Regarding references the RMSin the of clock the GLONASSproducts from satellite different clock, ACs although was considered the different

ISPRS Int. J. Geo-Inf. 2018, 7, 85 9 of 20 references in the clock products from different ACs was considered before comparison, all the products performed much worse than that of the GPS satellite except for the products from CLK93, leaving large ISPRS Int. J. Geo-Inf. 2018, 7, x FOR PEER REVIEW 9 of 19 room for improvement. In particular, the RMS of IGS03 was 9.38 ns, and even higher for R10 (26 ns). The reasonbefore maycomparison, be that all GLONASS the products adopts performed FDMA much and worse different than that ACs of adoptthe GPS different satellite except strategies for in dealingthe with products pseudo-range from CLK93, IFB leaving in resolving large room clock for improvement. corrections [In34 particul,35]. Inar, addition, the RMS of the IGS03 GLONASS was product9.38 from ns, CAS01and even is calculatedhigher for R10 by the(26 Institutens). The ofreason Geodesy may andbe that Geophysics GLONASS and adopts broadcasted FDMA and by the Academydifferent of Opto-Electronics. ACs adopt different However, strategies it is currently in dealing in thewith initial pseudo-range and test phaseIFB in and resolving the performance clock of thecorrections GLONASS [34,35]. product In addition, will be furtherthe GLONASS improved. product At from the same CAS01 time, is calculated the different by thegenerations Institute of of Geodesy and Geophysics and broadcasted by the Academy of Opto-Electronics. However, it is GLONASS satellite (GLONASS-M and GLONASS-KI) were analyzed by the same product, and we currently in the initial and test phase and the performance of the GLONASS product will be further also found nothing regular between them. improved. At the same time, the different generations of GLONASS satellite (GLONASS-M and GLONASS-KI) were analyzed by the same product, and we also found nothing regular between them. 4.3. Validation Results of BDS and Galileo Real-Time Orbit and Clock Products The4.3. real-timeValidation BDSResults and of BDS Galileo and Galileo orbit andReal-Time clock Orbit products and Clock are Products currently only broadcast by CLK93. The real-timeThe BDSreal-time and BDS Galileo and Galileo products orbit were and clock validated products by comparingare currently themonly broadcast with the by final CLK93. precise productThe “GBM” real-time released BDS and by Galileo GFZ. Theproducts validation were validated results of by the comparing BDS/Galileo them orbitwith the and final clock precise product are shownproduct in “GBM” Figures released6 and7 ,by respectively; GFZ. The validation and their results average of the accuracyBDS/Galileo during orbit and the clock test product period is illustratedare shown in Table in 5Figures. In the 6 testand period,7, respectively; the BDS and constellation their average consisted accuracy of during five GEO, the test six period IGSO- is and three MEO-satellites,illustrated in Table and 5. theIn the accuracies test period, of thethe real-timeBDS constellation or final orbitconsisted and of clock five productGEO, six forIGSO- those and GEO three MEO-satellites, and the accuracies of the real-time or final orbit and clock product for those satellites were generally much worse than those of other satellites due to the poor geometry of the GEO satellites were generally much worse than those of other satellites due to the poor geometry of GEO satellites tracked by the ground stations in orbit and clock determination. Thus, the results for the the GEO satellites tracked by the ground stations in orbit and clock determination. Thus, the results BDS IGSOfor the (C06–C10, BDS IGSO C13) (C06–C10, and MEO C13) (C11, and MEO C12, C14)(C11, satellitesC12, C14) were satellites only were considered only considered in our experiment. in our In viewexperiment. of the Galileo In view constellation, of the Galileo there constellation, are 17 satellites there are in 17 orbit, satellites of which in orbit, 15 satellitesof which 15 are satellites marked as availableare inmarked the ephemeris, as available and in the CLK93 ephemeris, estimates and theCLK93 orbit estimates and clock the only orbit for and those clock available only for satellites.those available satellites. Table 5. The average accuracies of the BeiDou Navigation Satellite System (BDS)/Galileo orbit and clock productsTable 5. The from average CLK93 accuracies when compared of the BeiDou with Navigation the GBM final Satellite product System during (BDS)/Galileo the test period.orbit and clock products from CLK93 when compared with the GBM final product during the test period.

OrbitsOrbits (cm)(cm) Clocks (ns) Clocks (ns) System System R RA A C C 1DRMS 1DRMS RMS STD RMS STD BDS BDS7.83 7.8314.75 14.75 18.8518.85 14.54 14.54 3.00 0.32 3.00 0.32 Galileo Galileo3.21 3.215.39 5.39 4.41 4.41 4.42 4.42 0.39 0.18 0.39 0.18

FigureFigure 6. The 6. RMS The ofRMS the of differences the differences of the of BDS the and BDS Galileo and Galileo orbit betweenorbit between the CLK93 the CLK93 real-time real-time product product and the GBM final product. and the GBM ﬁnal product.

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Figure 7. The RMS (blue bar) and STD (red bar) of the differences of BDS ( toptop)) and Galileo ( (bottom)) between the CLK 93 real-time product and thethe GBMGBM ﬁnalfinal product.product.

It can be seen from Figure 6 that the real-time Galileo orbit was more accurate than that of BDS, It can be seen from Figure6 that the real-time Galileo orbit was more accurate than that of BDS, but a bit worse than that of GPS, and the accuracy of the BDS MEO satellites was better than that of but a bit worse than that of GPS, and the accuracy of the BDS MEO satellites was better than that the BDS IGSO satellites. Regarding the real-time clock accuracy shown in Figure 7, the Galileo of the BDS IGSO satellites. Regarding the real-time clock accuracy shown in Figure7, the Galileo satellites also outperformed the BDS satellites. It can be found in Table 5 that the differences of BDS satellites also outperformed the BDS satellites. It can be found in Table5 that the differences of satellite orbits between the CLK93 real-time product and the GBM final product in the radial, along- BDS satellite orbits between the CLK93 real-time product and the GBM final product in the radial, track, and along-cross directions were individually 7.83, 14.75, and 18.85 cm. The average STD of real- along-track, and along-cross directions were individually 7.83, 14.75, and 18.85 cm. The average STD time BDS satellite clock product was 0.32 ns, and its average RMS reached 3.00 ns where the RMS of of real-time BDS satellite clock product was 0.32 ns, and its average RMS reached 3.00 ns where the MEO satellites was larger than that of the IGSO satellites. Moreover, the differences of the Galileo RMS of MEO satellites was larger than that of the IGSO satellites. Moreover, the differences of the satellite orbits between the real-time CLK93 product and the GBM final product in the radial, along- Galileo satellite orbits between the real-time CLK93 product and the GBM final product in the radial, track, and along-cross directions were 3.21, 5.39, and 4.41 cm, respectively. The average STD of clock along-track, and along-cross directions were 3.21, 5.39, and 4.41 cm, respectively. The average STD of correction was 0.18 ns, while the average RMS was 0.39 ns. All in all, the performance of the Galileo clock correction was 0.18 ns, while the average RMS was 0.39 ns. All in all, the performance of the satellite orbits and clocks was much better than that of BDS in the CLK93 product. The primary reason Galileo satellite orbits and clocks was much better than that of BDS in the CLK93 product. The primary is that the number of contributed ground stations that can track BDS satellites is much smaller than reason is that the number of contributed ground stations that can track BDS satellites is much smaller that of Galileo. than that of Galileo. 5. Positioning Results of Real-Time PPP with IGS Stations 5. Positioning Results of Real-Time PPP with IGS Stations The GNSS satellite orbit and clock products are generally used for the real-time precise point The GNSS satellite orbit and clock products are generally used for the real-time precise positioning. In this section, we further validated the performance of real-time products by the PPP point positioning. In this section, we further validated the performance of real-time products by the technique. The PPP software used in this experiment, named RTPosNavi_AOE (version 1.0.17a, PPP technique. The PPP software used in this experiment, named RTPosNavi_AOE (version 1.0.17a, Beijing, China), was developed by the Academy of Opto-Electronics (AOE) based on the open-source Beijing, China), was developed by the Academy of Opto-Electronics (AOE) based on the open-source software RTKLIB (version 2.4.3, Japan) [19]. The PPP rover used for this validation was conducted in software RTKLIB (version 2.4.3, Japan) [19]. The PPP rover used for this validation was conducted static and kinematic modes, respectively, but the data processing strategy was all designed as in static and kinematic modes, respectively, but the data processing strategy was all designed as kinematic mode, i.e., the estimated parameters of rover coordinates in two neighboring epochs were kinematic mode, i.e., the estimated parameters of rover coordinates in two neighboring epochs were considered as independent. The PPP observation model was the ionosphere-free (IF) combination of considered as independent. The PPP observation model was the ionosphere-free (IF) combination of dual-frequency carrier phase and pseudo-range observations, and the Kalman filter was adopted for dual-frequency carrier phase and pseudo-range observations, and the Kalman filter was adopted for parameter estimation. The specific processing strategy is listed in Table 6. To evaluate the accuracy parameter estimation. The specific processing strategy is listed in Table6. To evaluate the accuracy of real-time PPP, the real-time data stream in RTCM format from 10 stations from the IGS/MGEX of real-time PPP, the real-time data stream in RTCM format from 10 stations from the IGS/MGEX network was selected in this study. The distribution of the selected stations for the experiment is shown in Figure 8.

ISPRS Int. J. Geo-Inf. 2018, 7, 85 11 of 20

networkISPRS Int. J. was Geo-Inf. selected 2018, 7, inx FOR this PEER study. REVIEW The distribution of the selected stations for the experiment 11 of is19 shown in Figure8. Table 6. The processing strategy for precise point positioning (PPP) used for validating the Tableperformance 6. The processing of the real-time strategy GNSS for precise orbit and point clock positioning product. (PPP) used for validating the performance of the real-time GNSS orbit and clock product. Index Items Processing Strategies 1. Index Rover coordinates Items Real-time estimation; Processing initial value Strategies determined by SPP 2. 1. Ionospheric Rover coordinatesdelay Real-time Ionosphere-free estimation; initial combination value determined by SPP 3. 2. Satellite orbit Ionospheric and clock delay SSR corrections Ionosphere-free + Broadcast combination Ephemeris 4. 3. Receiver Satellite clock orbit and clock SSR corrections Real-time + estimation Broadcast Ephemeris 5. 4. Tropospheric Receiver delay clock Saastamoinen Real-timemodel + real-time estimation estimation 5. Tropospheric delay Saastamoinen model + real-time estimation

6. 6.Antenna Antenna PCO/PCV PCO/PCV igs14.atx igs14.atx 7. 7.Cut-off Cut-off elevation elevation angle angle 7°7 ◦ 8. 8.Ambiguity Ambiguity Float Float 9. 9. Cycle CycleSlip Slip Detected Detected by by MW MW and and GF GF

Figure 8. The distribution of the selected stations for the real-time PPPPPP testtest experiment.experiment.

5.1. Result of Real-Time PPP inin StaticStatic ModeMode InIn thisthis section,section, thethe selectedselected referencereference stationsstations werewere testedtested forfor staticstatic mode.mode. RTPosNavi_AOERTPosNavi_AOE receivedreceived thethe real-time real-time raw raw data data and and the GNSSthe GNSS satellite satellite orbit andorbit clock and products clock products via IGG-Ntrip via IGG-Ntrip software carriedsoftware out carried the PPP out result. the TakingPPP result. the case Taking of the CEDUthe case station of the as anCEDU example, station our as experiment an example, began our at 00:00:00experiment (UTC) began on 16at February00:00:00 (UTC) 2018 and on lasted16 February for 20 h.2018 The and time lasted series for of 20 the h. differences The time betweenseries of the PPP-estimateddifferences between and IGS-released the PPP-estimated coordinates and IGS-releas in the E (East-West),ed coordinates N (North-South), in the E (East-West), and U (Up-Down) N (North- directionsSouth), and are U shown(Up-Down) in Figure directions9, where are the shown convergence in Figure time 9, where was usuallythe convergence less than time 30 min, was and usually the positioningless than 30 accuracy min, and after the ﬁnishingpositioning the accuracy convergence after process finishing was the almost convergence better than process 10 cm was in the almost E, N, andbetter U than directions. 10 cm in the E, N, and U directions. Table7 7 shows shows the the average average positioning positioning accuracies accuracies of of the the static static mode mode in in the the E, E, N, N, and and U U directions directions during all testtest periodsperiods (without(without thethe convergenceconvergence time)time) forfor allall selectedselected stations.stations. RegardingRegarding 3D3D positioningpositioning accuracy, CLK93CLK93 hadhad thethe bestbest performanceperformance (about 4.18 cm), followed by CLK51, IGS03, CAS01, CLK70,CLK70, CLK80,CLK80, CLK10,CLK10, andand CLK20.CLK20. Thus, we concluded that the real-time PPP could achieve a positioning withwith anan accuracy ofof about 6 cm, provided one was able to obtain the real-time orbit and clockclock productsproducts fromfrom thethe currentcurrent eighteight selectedselected ACs.ACs.

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FigureFigure 9.9. TimeTime seriesseries ofof thethe differencesdifferences betweenbetween PPP-estimatedPPP-estimated (static(static mode)mode) andand IGS-releasedIGS-released coordinatescoordinates in in E E (East-West), (East-West), N N (North-South), (North-South), U U (Up-Down) (Up-Down) directions directions at at CEDU CEDU station. station. The The results results basedbased on on the the real-time real-time products products from from IGS03, IGS03, CLK10, CLK10, CLK20, CLK20, CLK51, CLK51, CLK70, CLK70, CLK80, CLK80, CLK93, CLK93, and CAS01 and areCAS01 individually are individually illustrated illustrated by each sub-ﬁgure.by each sub-figure.

Table 7. Static mode positioning RMS of the PPP-estimated result using the orbit and clock products Tablefrom 7.differentStatic mode ACs. positioning RMS of the PPP-estimated result using the orbit and clock products from differentDirection ACs. IGS BKG DLR ESA GFZ GMV CNES CAS Stations (cm) IGS03 CLK10 CLK20 CLK51 CLK70 CLK80 CLK93 CAS01 IGS BKG DLR ESA GFZ GMV CNES CAS Stations Direction (cm) E IGS03 1.94 CLK103.75 CLK203.09 CLK512.63 CLK703.16 CLK802.91 CLK932.04 CAS013.82 N 3.62 3.61 4.75 3.13 4.22 2.98 2.81 0.12 CEDU E 1.94 3.75 3.09 2.63 3.16 2.91 2.04 3.82 N 3.62 3.61 4.75 3.13 4.22 2.98 2.81 0.12 CEDU U 2.91 2.81 2.57 2.15 0.33 3.14 1.20 1.69 3DU 5.04 2.91 5.91 2.81 6.22 2.57 2.154.62 0.335.28 3.145.21 1.203.67 1.694.17 3D 5.04 5.91 6.22 4.62 5.28 5.21 3.67 4.17 E 2.01 3.82 3.15 3.61 3.02 3.19 2.11 2.66 E 2.01 3.82 3.15 3.61 3.02 3.19 2.11 2.66 N 3.64 3.64 3.65 3.65 4.78 0.990.99 3.023.02 4.214.21 2.852.85 3.163.16 AZU1AZU1 U 2.88 2.88 2.79 2.79 2.62 2.652.65 3.113.11 0.380.38 1.181.18 2.172.17 3D 5.06 5.97 6.30 4.59 5.28 5.30 3.74 4.67 3D 5.06 5.97 6.30 4.59 5.28 5.30 3.74 4.67 E 1.95 3.8 3.16 3.58 3.17 3.11 2.15 2.56 NE 1.95 3.59 3.713.8 3.164.72 1.143.58 3.363.17 3.113.11 2.952.15 3.292.56 RVDI UN 3.59 2.68 3.71 2.73 4.72 2.48 2.351.14 2.453.36 2.973.11 1.132.95 2.673.29 RVDI 3DU 2.68 4.89 2.73 5.97 2.48 6.20 4.432.35 5.232.45 5.312.97 3.821.13 4.952.67 3DE 4.89 1.81 5.97 3.88 6.20 3.12 3.534.43 3.055.23 3.25.31 2.013.82 2.594.95 N 3.49 3.55 4.3 1.04 3.03 4.06 3.26 3.25 ORID UE 1.81 2.43 3.88 2.18 3.12 2.32 2.713.53 3.093.05 0.69 3.2 2.03 2.01 2.542.59 3DN 3.49 4.62 3.55 5.69 5.80 4.3 4.57 1.04 5.293.03 5.224.06 4.333.26 4.873.25 ORID UE 2.43 1.77 2.18 3.6 2.32 3.22 3.622.71 3.023.09 3.260.69 2.482.03 2.72.54 N 3.25 3.41 4.13 1.11 3.68 3.69 3.51 3.37 STJO 3D 4.62 5.69 5.80 4.57 5.29 5.22 4.33 4.87 U 1.98 2.45 2.15 2.61 2.48 3.14 1.93 2.44 3DE 1.77 4.20 5.533.6 3.225.66 4.603.62 5.373.02 5.843.26 4.712.48 4.96 2.7 N 3.25 3.41 4.13 1.11 3.68 3.69 3.51 3.37 STJO E 2.15 3.41 3.37 3.56 3.12 3.09 2.72 2.46 NU 1.98 3.91 2.45 3.71 2.15 4.36 0.952.61 3.212.48 3.823.14 3.441.93 3.522.44 SALU 3DU 4.20 1.74 5.53 2.37 5.66 2.02 2.424.60 3.155.37 3.325.84 1.364.71 2.274.96 3D 4.79 5.57 5.87 4.41 5.47 5.93 4.59 4.86 E 2.15 3.41 3.37 3.56 3.12 3.09 2.72 2.46 N 3.91 3.71 4.36 0.95 3.21 3.82 3.44 3.52 SALU U 1.74 2.37 2.02 2.42 3.15 3.32 1.36 2.27 3D 4.79 5.57 5.87 4.41 5.47 5.93 4.59 4.86 E 1.66 3.25 3.12 3.47 3.06 3.15 2.25 2.32 N 3.66 3.83 4.17 1.32 4.2 4.36 3.33 3.61 CAS1 U 1.85 2.28 2.22 2.53 0.91 0.77 1.45 2.53 3D 4.42 5.52 5.66 4.49 5.28 5.43 4.27 4.98

ISPRS Int. J. Geo-Inf. 2018, 7, 85 13 of 20

Table 7. Cont.

E 1.66 3.25 3.12 3.47 3.06 3.15 2.25 2.32 N 3.66 3.83 4.17 1.32 4.2 4.36 3.33 3.61 CAS1 U 1.85 2.28 2.22 2.53 0.91 0.77 1.45 2.53 3D 4.42 5.52 5.66 4.49 5.28 5.43 4.27 4.98 E 1.73 3.02 3.21 3.33 3.16 3.01 2.07 2.23 N 3.71 3.99 4.02 1.25 3.69 3.14 3.02 3.85 CHOF U 1.92 2.02 2.05 2.81 1.83 3.18 1.55 2.55 3D 4.52 5.40 5.54 4.53 5.19 5.39 3.98 5.13 E 1.87 3.01 3.12 3.67 3.19 3.22 1.94 2.44 N 3.55 3.7 4.21 1.04 3.78 3.64 3.51 3.59 RIO2 U 1.68 1.93 2.13 2.36 1.91 3.02 1.64 2.47 3D 4.35 5.15 5.66 4.49 5.30 5.72 4.33 4.99 E 1.91 3.32 3.17 3.51 3.16 3.2 1.83 2.32 N 3.46 3.51 3.66 1.23 3.45 3.34 3.57 3.45 HARB U 1.82 1.73 2.00 2.87 2.01 2.95 1.77 2.29 3D 4.32 5.17 5.26 4.70 5.09 5.50 4.36 4.79 E 1.88 3.49 3.17 3.45 3.11 3.13 2.16 2.61 N 3.59 3.67 4.31 1.32 3.56 3.64 3.23 3.12 Average U 2.19 2.33 2.26 2.55 2.13 2.36 1.52 2.36 3D 4.62 5.59 5.82 4.54 5.28 5.48 4.18 4.84

5.2. Result of Real-Time PPP in Simulated Kinematic Mode In this section, the selected reference stations were tested for simulated kinematic mode. Since the processing strategy was designed as a kinematic mode, the positioning result with the data from a static station can be generally considered as the best performance the real kinematic mode can achieve. This is because the effect of cycle slip, multi-path, and the interruption of signals can be reduced as much as possible. RTPosNavi_AOE receives the real-time raw data and the GNSS satellite orbit and clock products via the IGG-Ntrip software and carries out the PPP result. Taking CEDU station as an example, our experiment began at 00:00:00 (UTC) on 12 September 2017 and lasted for 20 h. IGS has released the known coordinates of every station and we directly used them as the reference for validating the positioning accuracy. The time series of the differences between the PPP-estimated and IGS-released coordinates in the E, N, and U directions are shown in Figure 10, where the results based on the different real-time GNSS satellite orbit and clock products are individually illustrated by each sub-figure. The ranges of the vertical axis of each sub-figure were set as the same for convenience during comparison, and the horizontal axis was the hour from the experiment start time. With the exception of the real-time product from IGS03 and CLK80, the convergence time was usually less than 30 min, and the positioning accuracy after finishing the convergence process was almost better than 15 cm in the E, N, and U directions. Table8 shows the average positioning accuracies of kinematic mode in the E, N, and U directions during all test periods (without the convergence time). The positioning result based on CNES CLK93 and ESA CLK51 had the best accuracy in the horizontal (about 4 cm) and vertical (about 6 cm) directions, respectively. Regarding 3D positioning accuracy, the CLK51 and CLK93 also had the best performance (about 6.90 cm), followed by CLK10, CLK20, IGS03, CLK80, CLK70, and CAS01. It should be noted that the number of systems provided by different ACs was different, only the product from CNES CLK93 included all GPS, BDS, GLONASS, and Galileo systems. Nevertheless, we concluded that real-time PPP could achieve a positioning with an accuracy of about 10 cm provided one was able to obtain the real-time orbit and clock products from the current eight selected ACs. ISPRS Int. J. Geo-Inf. 2018, 7, 85 14 of 20 ISPRS Int. J. Geo-Inf. 2018, 7, x FOR PEER REVIEW 14 of 19

FigureFigure 10. 10. Time series of of the the differences differences between between PPP- PPP-estimatedestimated (simulated (simulated kinematic kinematic mode) mode) and andIGS- IGS-releasedreleased coordinates coordinates in inE (East-West), E (East-West), N N (North-S (North-South),outh), U U (Up-Down) (Up-Down) direct directionsions at CEDUCEDU station.station. TheThe results results based based on on the the real-time real-time products products from from IGS03, IGS03, CLK10, CLK10, CLK20, CLK20, CLK51, CLK51, CLK70, CLK80,CLK70, CLK93,CLK80, andCLK93, CAS01 and are CAS01 individually are individually illustrated illustrated by each sub-ﬁgure. by each sub-figure.

Table 8. Simulated kinematic mode Positioning RMS of the PPP-estimated result using the orbit and Tableclock 8.productsSimulated from kinematic different mode ACs. Positioning RMS of the PPP-estimated result using the orbit and clock productsDirection from different IGS ACs. BKG DLR ESA GFZ GMV CNES CAS Stations (cm) IGS03 CLK10 CLK20 CLK51 CLK70 CLK80 CLK93 CAS01 IGS BKG DLR ESA GFZ GMV CNES CAS Stations Direction (cm) E IGS03 3.36 CLK102.74 CLK202.24 CLK513.78 CLK706.02 CLK804.89 CLK931.75 CAS015.03 N 3.86 2.51 1.89 1.96 3.76 2.60 1.67 2.57 CEDU E 3.36 2.74 2.24 3.78 6.02 4.89 1.75 5.03 NU 5.95 3.86 6.68 2.51 1.897.00 1.965.08 3.767.12 2.608.31 1.676.36 2.579.53 CEDU 3DU 7.85 5.95 7.65 6.68 7.007.59 5.086.62 7.1210.05 8.319.99 6.366.81 9.5310.86 3D 7.85 7.65 7.59 6.62 10.05 9.99 6.81 10.86 E 5.56 3.43 2.48 3.84 5.81 3.72 2.01 3.95 E 5.56 3.43 2.48 3.84 5.81 3.72 2.01 3.95 NN 5.84 5.84 2.02 2.02 1.96 2.642.64 1.861.86 2.602.60 1.621.62 2.282.28 AZU1AZU1 UU 6.44 6.44 6.08 6.08 6.87 4.984.98 7.327.32 7.537.53 6.136.13 8.538.53 3D 10.31 10.31 7.27 7.27 7.56 6.826.82 9.539.53 8.798.79 6.656.65 9.679.67 EE 4.83 4.83 3.39 3.39 2.88 2.562.56 4.994.99 4.194.19 4.454.45 4.814.81 N 4.71 2.34 1.86 1.78 3.24 2.58 2.03 3.22 RVDI N 4.71 2.34 1.86 1.78 3.24 2.58 2.03 3.22 RVDI U 6.02 6.71 6.54 5.99 6.99 7.76 5.33 8.56 3DU 6.02 9.04 6.71 7.87 7.386.54 6.755.99 9.186.99 9.197.76 7.235.33 10.338.56 3DE 9.04 5.17 7.87 2.68 2.497.38 2.816.75 5.129.18 4.139.19 3.037.23 4.1110.33 N 4.06 1.93 2.04 1.90 3.99 2.54 2.52 2.48 ORID UE 5.17 5.98 2.68 5.99 7.132.49 5.882.81 7.415.12 8.494.13 4.893.03 9.674.11 3DN 4.06 8.88 1.93 6.84 7.822.04 6.791.90 9.853.99 9.782.54 6.282.52 10.792.48 ORID EU 5.98 4.41 5.99 3.16 2.377.13 3.785.88 3.847.41 5.668.49 1.824.89 4.819.67 N 4.51 1.70 1.98 2.07 2.64 3.79 1.55 2.85 STJO 3D 8.88 6.84 7.82 6.79 9.85 9.78 6.28 10.79 U 5.79 7.33 6.96 6.01 8.73 7.42 6.43 9.34 3DE 4.41 8.56 3.16 8.16 7.342.37 7.403.78 9.893.84 10.075.66 6.861.82 10.894.81 N 4.51 1.70 1.98 2.07 2.64 3.79 1.55 2.85 STJO E 5.47 2.22 3.73 2.75 4.91 4.32 3.48 5.16 NU 5.79 4.06 7.33 1.85 2.866.96 1.886.01 3.678.73 2.407.42 2.336.43 2.639.34 SALU 3DU 8.56 6.07 8.16 6.78 6.447.34 5.637.40 7.419.89 8.6110.07 5.336.86 8.9810.89 3D 9.12 7.37 7.97 6.54 9.62 9.92 6.78 10.68 E 5.47 2.22 3.73 2.75 4.91 4.32 3.48 5.16 N 4.06 1.85 2.86 1.88 3.67 2.40 2.33 2.63 SALU U 6.07 6.78 6.44 5.63 7.41 8.61 5.33 8.98 3D 9.12 7.37 7.97 6.54 9.62 9.92 6.78 10.68 E 4.77 3.19 2.45 4.01 6.13 4.94 2.12 4.26 N 3.62 2.21 1.78 1.88 2.65 2.59 1.83 2.52 CAS1 U 5.11 6.93 6.89 5.68 8.03 7.81 6.16 9.83 3D 7.87 7.94 7.53 7.20 10.44 9.60 6.77 11.01 CHOF E 5.02 3.68 4.57 3.13 5.24 3.99 2.86 3.91

ISPRS Int. J. Geo-Inf. 2018, 7, 85 15 of 20

Table 8. Cont.

E 4.77 3.19 2.45 4.01 6.13 4.94 2.12 4.26 N 3.62 2.21 1.78 1.88 2.65 2.59 1.83 2.52 CAS1 U 5.11 6.93 6.89 5.68 8.03 7.81 6.16 9.83 3D 7.87 7.94 7.53 7.20 10.44 9.60 6.77 11.01 E 5.02 3.68 4.57 3.13 5.24 3.99 2.86 3.91 N 5.94 1.74 2.05 1.99 2.84 2.06 1.71 2.00 CHOF U 6.74 6.89 5.72 5.98 8.01 8.57 6.73 10.06 3D 10.29 8.00 7.60 7.04 9.98 9.67 7.51 11.71 E 4.33 2.89 2.92 3.23 5.53 4.73 2.11 4.74 N 5.15 2.10 1.76 2.34 2.35 1.83 1.97 1.74 RIO2 U 6.54 6.76 7.09 5.46 7.24 7.94 6.88 8.67 3D 9.38 7.64 7.87 6.76 9.41 9.42 7.46 10.03 E 4.68 3.14 2.48 4.01 5.88 5.01 2.12 5.65 N 5.51 2.24 1.96 1.91 3.69 3.08 2.01 2.25 HARB U 6.02 6.40 6.79 6.08 7.37 7.76 6.09 8.84 3D 9.41 7.47 7.49 7.52 10.12 9.99 6.75 10.72 E 4.76 3.05 2.86 3.39 5.35 4.56 2.58 4.64 N 4.73 2.06 2.02 2.04 3.07 2.61 1.92 2.45 Average U 6.07 6.65 6.74 5.67 7.56 8.02 6.03 8.84 3D 9.07 7.62 7.62 6.94 9.81 9.64 6.91 10.67

6. Positioning Result of Real-Time Kinematic PPP in Urban Experiment In this section, we conducted a real kinematic test in the cities of Beijing, Tianjin, and Shijiazhuang in China for a better understanding of the impact of real-time orbit and clock products from different ACs on the accuracy of PPP. Based on the analysis result, we found that the performance of the real-time GPS and BDS orbit (CLK93) from CNES was the best one among the eight selected ACs and is the only center that can simultaneously broadcast GPS, BDS, GLONASS, and Galileo products. Moreover, the BDS orbit and clock product began broadcasting by CAS01 from October 2017 in the test phase. Thus, the real-time satellite orbit and clock product from CLK93 and CAS01 were only selected in this experiment, as the test result could indicate the different performances of the products from those two ACs. The terminal used for this real-time kinematic PPP experiment was assembled by integrating a NovAtel GNSS receiver board 618 and an Advanced RISC Machines (ARM) for the PPP calculation; a 4G mobile communication module was used for receiving the real-time orbit and clock product from the servers. The NovAtel GNSS receiver board can track the GPS, BDS, GLONASS, and Galileo signals and output the raw observations with an interval of 1 s. The PPP software RTPosNavi_AOE was installed on the corresponding ARM and the positioning result was sent to our servers for showing on a precise map. The terminal and precise map are given in Figure 11. Moreover, we used two terminals in our test, one was only for the GPS mode and the other for the GPS+BDS mode. Two copies of RTPosNavi_AOE were run on the ARM to use the real-time products from CLK93 and CAS01, respectively. To obtain the real coordinates for the positioning accuracy validation, the IGS ﬁnal orbit and clock product were introduced for PPP resolution in a post-processing mode. ISPRS Int. J. Geo-Inf. 2018, 7, x FOR PEER REVIEW 15 of 19

N 5.94 1.74 2.05 1.99 2.84 2.06 1.71 2.00 U 6.74 6.89 5.72 5.98 8.01 8.57 6.73 10.06 3D 10.29 8.00 7.60 7.04 9.98 9.67 7.51 11.71 E 4.33 2.89 2.92 3.23 5.53 4.73 2.11 4.74 N 5.15 2.10 1.76 2.34 2.35 1.83 1.97 1.74 RIO2 U 6.54 6.76 7.09 5.46 7.24 7.94 6.88 8.67 3D 9.38 7.64 7.87 6.76 9.41 9.42 7.46 10.03 E 4.68 3.14 2.48 4.01 5.88 5.01 2.12 5.65 N 5.51 2.24 1.96 1.91 3.69 3.08 2.01 2.25 HARB U 6.02 6.40 6.79 6.08 7.37 7.76 6.09 8.84 3D 9.41 7.47 7.49 7.52 10.12 9.99 6.75 10.72 E 4.76 3.05 2.86 3.39 5.35 4.56 2.58 4.64 N 4.73 2.06 2.02 2.04 3.07 2.61 1.92 2.45 Average U 6.07 6.65 6.74 5.67 7.56 8.02 6.03 8.84 3D 9.07 7.62 7.62 6.94 9.81 9.64 6.91 10.67

6. Positioning Result of Real-Time Kinematic PPP in Urban Experiment In this section, we conducted a real kinematic test in the cities of Beijing, Tianjin, and Shijiazhuang in China for a better understanding of the impact of real-time orbit and clock products from different ACs on the accuracy of PPP. Based on the analysis result, we found that the performance of the real-time GPS and BDS orbit (CLK93) from CNES was the best one among the eight selected ACs and is the only center that can simultaneously broadcast GPS, BDS, GLONASS, and Galileo products. Moreover, the BDS orbit and clock product began broadcasting by CAS01 from October 2017 in the test phase. Thus, the real-time satellite orbit and clock product from CLK93 and CAS01 were only selected in this experiment, as the test result could indicate the different performances of the products from those two ACs. The terminal used for this real-time kinematic PPP experiment was assembled by integrating a NovAtel GNSS receiver board 618 and an Advanced RISC Machines (ARM) for the PPP calculation; a 4G mobile communication module was used for receiving the real-time orbit and clock product from the servers. The NovAtel GNSS receiver board can track the GPS, BDS, GLONASS, and Galileo signals and output the raw observations with an interval of 1 s. The PPP software RTPosNavi_AOE was installed on the corresponding ARM and the positioning result was sent to our servers for showing on a precise map. The terminal and precise map are given in Figure 11. Moreover, we used two terminals in our test, one was only for the GPS mode and the other for the GPS+BDS mode. Two copies of RTPosNavi_AOE were run on the ARM to use the real-time products from CLK93 and ISPRSCAS01, Int. respectively. J. Geo-Inf. 2018, To7, 85 obtain the real coordinates for the positioning accuracy validation, the16 ofIGS 20 final orbit and clock product were introduced for PPP resolution in a post-processing mode.

Figure 11. The positioning terminal (left) and the precise map (right) used for the real-time kinematic Figure 11. The positioning terminal (left) and the precise map (right) used for the real-time kinematic PPP test. PPP test.

The test was carried out in Beijing, Tianjin, and Shijiazhuang individually during the periods of 10:00:00The to test 12:30:00 was carried (UTC) out on in22 Beijing, October Tianjin, 2017, 15:40:00 and Shijiazhuang to 17:00:00 individually (UTC) on 24 during October the 2017 periods and of 10:00:00 to 12:30:00 (UTC) on 22 October 2017, 15:40:00 to 17:00:00 (UTC) on 24 October 2017 and ISPRS13:00:00 Int. J.to Geo-Inf. 14:40:00 2018 (UTC), 7, x FOR on PEER 26 October REVIEW 2017. The maximum speed of the experimental vehicle 16 wasof 19 13:00:00 to 14:40:00 (UTC) on 26 October 2017. The maximum speed of the experimental vehicle was about 8080 km/h.km/h. TheThe experiment experiment routes routes in thesein these three th citiesree cities included included the urban, the urban, expressway, expressway, and ﬂyover and environmentflyover environment and are and individually are individually displayed displayed in Figure in Figure12. The 12. distances The distances of these of threethese three routes routes were aboutwere about 23.5, 34.2,23.5, and34.2, 26.1 and km, 26.1 respectively. km, respectively.

Figure 12. The road selected for our experiment in Beijing (left), Tianjin (middle), and Shijiazhuang (right). Figure 12. The road selected for our experiment in Beijing (left), Tianjin (middle), and Shijiazhuang (right).

Figure 13 shows the time series of the differences between the real-time PPP-estimated and post PPP Figureat the different 13 shows cities the in time the series E (East-West), of the differences N (North-South), between and the U real-time (Up-Down) PPP-estimated directions. andThe postresults PPP were at thebased different on the cities real-time in the products E (East-West), from CLK93 N (North-South), where the convergence and U (Up-Down) time was directions. usually Thealso resultsless than were 30 min, based and on the the positioning real-time products accuracy from after CLK93 finishing where the the convergence convergence process time waswas usuallyalmost alsobetter less than than 0.5 30 m min, in all and directions. the positioning The accuracy accuracy of after the ﬁnishingreal-time thekinematic convergence PPP result process based was almoston the betterproducts than from 0.5 CLK93 m in all and directions. CAS01 in The GPS accuracy and GPS of + the BDS real-time modes are kinematic illustrated PPP in result Table based9, where on the productsresult during from the CLK93 convergence and CAS01 period in GPS is not and remov GPS +ed. BDS It was modes found are that illustrated the positioning in Table9 ,accuracy where the of resultthe simulated during thekinematic convergence PPP decreased period is notsignificantly removed. wi Itth was respect found to that the the static positioning result in accuracySection 5.2 of theregardless simulated of which kinematic product PPP was decreased used. The signiﬁcantly average accuracies with respect of the to real-time the static kinematic result in SectionPPP based 5.2 regardlesson the CLK93 of which product product were about was used. 0.45–0.65 The average m and 0.75–1.5 accuracies m in of the the horizontal real-time kinematic and vertical PPP directions, based on therespectively, CLK93 product whereas were they about were 0.45–0.65 about 0.50–0.65 m and 0.75–1.5 m and m 0.70–1.20 in the horizontal m for CAS01, and vertical except directions, for the respectively,horizontal accuracy whereas in they Tianjin. were The about reason 0.50–0.65 why mthe and positioning 0.70–1.20 mresult for CAS01,based on except CAS01 for in the Tianjin horizontal was accuracymuch worse in Tianjin. than that The in reason other cities why theneeds positioning to be further result investigated. based on CAS01 in Tianjin was much worse than that in other cities needs to be further investigated.

Figure 13. Time series of the differences between real-time PPP-estimated and post PPP at Beijing (left), Tianjin (middle), and Shijiazhuang (right) in the E, N, and U directions. The results of GPS (top) and GPS + BDS (bottom) based on the real-time products from CLK93 are only shown here.

Moreover, comparing the GPS result with that of GPS + BDS, the positioning accuracies were all slightly improved in the horizontal and vertical directions regardless of which product was used. The average accuracies in the Beijing, Tianjin, and Shijiazhuang areas are also given in Table 9. We found that the real-time kinematic PPP could achieve the positioning with an accuracy of better than 0.6 and 1.0 m in the horizontal and vertical directions based on the real-time orbit and clock product from CLK93, while they were 0.95 and 1.0 m for CAS01. The product from CAS needs to be improved further.

ISPRS Int. J. Geo-Inf. 2018, 7, x FOR PEER REVIEW 16 of 19

about 80 km/h. The experiment routes in these three cities included the urban, expressway, and flyover environment and are individually displayed in Figure 12. The distances of these three routes were about 23.5, 34.2, and 26.1 km, respectively.

Figure 12. The road selected for our experiment in Beijing (left), Tianjin (middle), and Shijiazhuang (right).

Figure 13 shows the time series of the differences between the real-time PPP-estimated and post PPP at the different cities in the E (East-West), N (North-South), and U (Up-Down) directions. The results were based on the real-time products from CLK93 where the convergence time was usually also less than 30 min, and the positioning accuracy after finishing the convergence process was almost better than 0.5 m in all directions. The accuracy of the real-time kinematic PPP result based on the products from CLK93 and CAS01 in GPS and GPS + BDS modes are illustrated in Table 9, where the result during the convergence period is not removed. It was found that the positioning accuracy of the simulated kinematic PPP decreased significantly with respect to the static result in Section 5.2 regardless of which product was used. The average accuracies of the real-time kinematic PPP based on the CLK93 product were about 0.45–0.65 m and 0.75–1.5 m in the horizontal and vertical directions, respectively, whereas they were about 0.50–0.65 m and 0.70–1.20 m for CAS01, except for the horizontal accuracy in Tianjin. The reason why the positioning result based on CAS01 in Tianjin was ISPRS Int. J. Geo-Inf. 2018, 7, 85 17 of 20 much worse than that in other cities needs to be further investigated.

FigureFigure 13. 13.Time Time series series of theof the differences differenc betweenes between real-time real-time PPP-estimated PPP-estimated and post and PPP post at PPP Beijing at Beijing (left), Tianjin(left), (Tianjinmiddle (),middle and Shijiazhuang), and Shijiazhuang (right) in(right the) E, in N, the and E, N, U and directions. U directions. The results The results of GPS of ( topGPS) and(top) GPSand + GPS BDS + (BDSbottom (bottom) based) based on the on real-time the real-time products products from CLK93from CLK93 are only are shown only shown here. here.

Moreover, comparing the GPS result with that of GPS + BDS, the positioning accuracies were all Moreover, comparing the GPS result with that of GPS + BDS, the positioning accuracies were slightly improved in the horizontal and vertical directions regardless of which product was used. The all slightly improved in the horizontal and vertical directions regardless of which product was used. average accuracies in the Beijing, Tianjin, and Shijiazhuang areas are also given in Table 9. We found The average accuracies in the Beijing, Tianjin, and Shijiazhuang areas are also given in Table9. that the real-time kinematic PPP could achieve the positioning with an accuracy of better than 0.6 and We found that the real-time kinematic PPP could achieve the positioning with an accuracy of better 1.0 m in the horizontal and vertical directions based on the real-time orbit and clock product from than 0.6 and 1.0 m in the horizontal and vertical directions based on the real-time orbit and clock CLK93, while they were 0.95 and 1.0 m for CAS01. The product from CAS needs to be improved further. product from CLK93, while they were 0.95 and 1.0 m for CAS01. The product from CAS needs to be improved further.

Table 9. The accuracies of real-time kinematic PPP using the CLK93 and CAS01 in GPS and GPS + BDS modes.

Accuracy (m) City Product System Available Epochs Horizontal Vertical GPS 0.47 0.78 8005 CLK93 GPS + BDS 0.46 0.73 8023 Beijing GPS 0.60 0.77 8005 CAS01 GPS + BDS 0.56 0.74 8023 GPS 0.63 1.06 4615 CLK93 GPS + BDS 0.51 0.50 4758 Tianjin GPS 1.38 0.92 4615 CAS01 GPS + BDS 1.29 0.89 4758 GPS 0.62 1.14 5500 CLK93 GPS + BDS 0.53 0.83 5591 Shijiazhuang GPS 0.60 1.16 5500 CAS01 GPS + BDS 0.53 1.08 5591 GPS 0.58 1.01 18,120 CLK93 GPS + BDS 0.50 0.70 18,372 Average GPS 0.94 0.96 18,120 CAS01 GPS + BDS 0.87 0.91 18,372 ISPRS Int. J. Geo-Inf. 2018, 7, 85 18 of 20

7. Conclusions The SSR product including the real-time orbit and clock corrections is an essential and indispensable product for the GPS/BDS/GLONASS/Galileo PPP user achieving precise point positioning in real-time mode. In this study, a brief introduction of the methods to use real-time products for correcting satellite orbit and clock errors was summarized and the accuracy of real-time orbit and clock products from eight selected ACs were validated by using IGS, ESA, and GBM precise products as references. Further insights into PPP performance were provided by using static data to simulate kinematic positioning and real-time kinematic tests. The main conclusions are presented as follows:

1. A comparison of real-time precise products developed by eight selected ACs, i.e., BKG, DLR, ESA, GFZ, GMV, CNES, CAS, and IGS, with respect to the final precise products, showed that the RMS of differences for GPS between real-time orbit and IGS final orbit was better than 8 cm, whereas the STD of GPS clock differences was better than 0.3 ns. The RMS of GLONASS orbit differences between the real-time product and ESA final product was less than 10 cm, and clock STD was better than 0.6 ns, except for CAS01. The 1DRMS of orbit differences for BDS and Galileo in the CLK93 product were 14.54 and 4.42 cm, respectively, in contrast to 0.32 and 0.18 ns for clock STD. 2. Static mode and simulated kinematic mode PPP results from 10 stations in the IGS/MGEX network indicated that the PPP results using real-time products developed by BKG, DLR, ESA, GFA, GMV, CNES, and CAS needed nearly the same convergence time (about 20–30 min) to reach about 10 cm and 15 cm level accuracy as using IGS final precise ephemeris and clock products. 3. CNES and CAS products were selected for the real-time kinematic PPP tests carried out in Beijing, Tianjin, and Shijiazhuang, China. Results indicated that CLK93 products showed a higher accuracy than CAS01 in real-time kinematic PPP, horizontally and vertically, when being tested under the same conditions. Moreover, when using the same product for real-time PPP solution, the GPS + BDS dual system showed higher accuracy than the GPS single system. 4. The real-time SSR product from the eight selected ACs was advantageous to aid the real-time PPP user achieving positioning with sub-meter level accuracy (including the result during the convergence or re-convergence period) in the horizontal direction and could be used for lane identification in the near future.

It must be pointed out that the tests conducted in this paper were only based on one-week data from real-time products. Data from a longer timeframe will be used for the follow-up research, where real-time data from reference stations in different locations and different periods will be collected for systematic assessment and analysis of real-time product performance.

Acknowledgments: Partially funded by the National Natural Science Foundation of China (41674043, 41704038, 41730109), the National Key Research Program of China “Collaborative Precision Positioning Project” (No. 2016YFB050190), Beijing Nova program (xx2017042), Beijing Natural Science Foundation (8184092) and Beijing youth talent support program (2017000021223ZK13). The third author acknowledges the financial support from the China Scholarship Council. Thanks also to the IGG-Ntrip software provided by the Institute of Geodesy and Geophysics, Chinese Academy of Sciences, the original observations provided by IGS, and the multi-system SSR products provided by the analysis centers such as BKG, GMV, and CNES. Author Contributions: Zishen Li and Liang Wang conceived and designed the experiments; Zhiyu Wang performed the experiments, analyzed the data, and wrote the paper; Liang Wang also helped to analyze the data; Zishen Li, Liang Wang, Xiaoming Wang, and Hong Yuan helped to improve the manuscript. Conflicts of Interest: The authors declare no conflict of interest.

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